1. Field of the Invention
The present invention relates generally to steel pistons and to methods of forming steel pistons.
2. Related Art
Many conventional monobloc piston bodies are made of Aluminum and include a crown portion, a pair of skirt portions, a pair of pin bosses and a plurality of pin boss bridges which extend between the skirt portions and the pin bosses. Such monobloc piston bodies are typically initially formed to a rough shape through a casting or forging process and subsequently machined to their final shapes. The machining operations usually start by locating a datum or reference point on an as-cast or as-forged feature on the piston and machining an inner portion of a lower end (but not the entire lower end) of each piston skirt to present a counter-bore with a generally flat surface. Once these counter-bores are formed, they are used as reference and/or datum points for many, if not all, of the subsequent machining processes to the piston body and also for final inspection of the piston body to ensure that the piston body can perform its intended function.
Many modern engine manufacturers employ advanced technologies, such as direct injection and turbo-charging to improve performance and fuel economy. As a consequence, these and other advanced technologies often result in increased combustion temperatures and pressures which could damage Aluminum pistons. To resist these increased combustion temperatures and pressures, some piston manufacturers have taken to manufacturing pistons of steel rather than Aluminum. Because of the increased strength of steel as compared to Aluminum, it is desirable to form pistons with thinner skirt and pin boss bridge wall thicknesses for weight reduction purposes. However, any reduction in the wall thickness of the skirt typically has the negative effect of reducing the material from which to machine the counter-bore surfaces, thereby making certain machining operations more difficult. Some piston manufacturers have attempted to machine counter-bore surfaces into skirts with reduced wall thicknesses. Such a piston body with counter-bore surfaces machined into its skirts is shown in FIGS. 1 and 2. However, this approach may result in outer, non-machined portions which are extremely thin, brittle and razor-sharp and could cause be easily damaged during the subsequent machining operations or during handling prior to or after the machining operations. In order to avoid these drawbacks, some manufacturers produce steel pistons with skirts that are thicker than is structurally necessary just to provide the skirts with enough thickness from which to machine the counter-bore surfaces without creating thin, brittle and sharp outer, non-machined surfaces on the skirts.